The deposition of contaminants on process equipment, screens, and containment vessels in papermaking significantly reduces process efficiency and paper quality. Deposits on machine wires, felts, foils, headbox surfaces, screens, and instruments require costly downtime for cleaning to avoid the problems associated with poor process control, reduced throughput, and substandard sheet properties. These contaminants are generically referred to in the paper industry as either “pitch” or “stickies”.
Pitch deposits generally originate from natural resins (fatty acids, rosin acids, sterols, etc), resin salts, coating binders, or sizing agents found in the pulp (Journal of Pulp and Paper Science, 22:11,431-439, 1996). Pitch may also contain inorganic materials such as talc, calcium carbonate, or titanium dioxide. Stickies generally refers to the hydrophobic substances used in the manufacture of paper such as pressure sensitive or contact adhesives such as styrene butadiene rubber, hot melt adhesives such as ethylene vinyl acetate and thermoplastics inks (styrene acrylate, polyvinyl butyral) that can form deposits when reintroduced in recycled fiber systems. Other common hydrophobic contaminants found in recycle applications include wax, which originates primarily from wax-coated old corrugated containers, and polyisoprene.
Recycled fiber refers to secondary fibers which are repulped to provide the papermaking furnish with raw material for the production of new papers. The secondary fibers may be either pre-consumer or post-consumer paper material that is suitable for use in the production of paper products. Sources of secondary fiber may include old newspaper (ONP), old corrugated containers (OCC), mixed office waste (MOW), computer printout (CPO), ledger, etc. These once-processed papers contain various types of adhesives (pressure sensitive, hot melts, etc.), inks, and coating binders. Coating binders can include the so-called ‘white pitch’ associated with the repulping of polyvinyl acetate and styrene butadiene coated broke.
Pitch and stickies are hydrophobic in nature and thus quite unstable as colloids in aqueous papermaking environments, thereby facilitating their deposition. The major problems arising from deposition are as follows: (1) reduced throughput due to plugging of forming fabrics and press felts (2) sheet holes or paper breaks due to large deposits breaking loose from the equipment, and (3) reduced sheet quality due to contaminants incorporated in the final sheet.
A current protocol for dealing with contaminant deposition in the papermaking process involves the addition of talc and saponite clay. A more improved methodology is needed.